1. Field of the Invention
This invention relates to a process wherein a double-solvent treatment is used for the selective recovery of hydrocarbons and organic materials from tar sands. The process of this invention is particularly adapted to separate such hydrocarbons and organic constituents, hereinafter referred to as organic constituents, from aggregates in which mineral particles are discrete and comprise the major portion of the naturally-occurring aggregate, in order to recover virtually solid-free materials which may be further treated in standard refinery operations. It is to be distinguished from those treatment of natural asphalt in which only coarsest particles of sand or minerals are removed, leaving finely divided clays or other minerals in the extract. The process is also to be distinguished from treatments for the recovery of oil from consolidated materials, such as shale. According to the process of this invention, hydrocarbons and organic materials are selectively extracted from tar sands by a mixture of light naphtha and methanol. The extraction results in rapid collapse of the aggregate and the simultaneous separation of the mixture into three phases, which separation is brought about by trace amounts of water. The most desirable, non-polar components of the extract are recovered from the naphtha phase; more polar soluble components are recovered from the methanol phase; and the majority of the less desirable asphaltenes remain insoluble but are separated from the sand and are found at the interface of the two solvents. These asphaltenes may be recovered from the interface or may be removed by filtration of the lower methanol layer through the sand.
2. Description of the Prior Art
As sources of crude oil become less available, there is a greater need to develop known but less easily handled petroleum resources. One such resources is tar sand, a sandy material having both water and tar within its interstices. Large deposits are found in various parts of the world. For example, one of the largest known deposits of tar sands lies in the Athabasca district of Alberta Canada, and extends for many thousands of square miles.
These sands contain tremendous reserves of hydrocarbon constituents. For example, the oil in the sands may vary from about 5 to 21% by volume, generally in the range of about 12% by volume. The gravity of the oil ranges from about 6.degree. to 10.degree. API, generally about 8.degree. API. The tar sand beds may range from about 100 to 400 feet thick, covered by an overburden from about 200 to 300 feet. A typical oil recovered from the sands has an initial boiling point of about 300.degree. F., 1.0% distillation to 430.degree. F., 20% distillation to 650.degree. F., and 50.0% distillation to 980.degree. F. Tar sands represent a significant petroleum resource which may ease the growing shortage of sources available to satisfy our petroleum demand. However, before the petroleum material in the tar sand can be employed in ordinary oil refining operaton, it must be separated from the solid, sandy material.
Thus, tar sands suffer the disadvantage of requiring additional processing steps over conventional forms of oil recovery. The high cost of separating tar from sand has been the greatest restriction on the use of tar sand as an economical source of crude petroleum. It is, therefor, essential to the commercial feasibility of any process for the recovery of hydrocarbons from tar sands that its costs be low while maintaining a high recovery of hydrocarbons.
The peculiar nature of the tar sand aggregate has heretofore frustrated atempts to attain these goals, though, some are relatively soft and free-flowing while others are very hard and rocklike. For example, the tar sands of the Athabasca district are composed of an almost pure silica sand, each grain of which is surrounded by a layer of water which, in turn, is surrounded by a film of oil. The oil film may also contain significant quantitites of clay. The interstices between the grains are largely filled with oil. This construction of the aggregate is believed to exist because the water content of the tar sands has a greater surface tension than the oil. Although, it is recognized that the exact compositions of the tar sands vary, even in an immediate location, the foregoing construction of the tar sand aggregate is apparently a general characteristic of the sands.
Various methods have been prepared for the recovery of hydrocarbons from tar sands, including direct fluid coking and retorting. These thermal processes are uneconomical due to the fact that the heat imparted to the sand cannot be effectively and efficiently recovered therefrom. Accordingly, attention has more recently focused on solvent extraction as a general method which may provide a commercially feasible process for the treatment of tar sands.
U.S. Pat. No. 2,825,677 disclosed a two-stage process wherein a hydrocarbon diluent, having a lesser specific gravity than that of the tar-sand oil, is first added to the sand to dilute the oil phase and reduce its viscosity, thereby making it more mobile and susceptible to the stopping forces of the aqueous phase. The specific gravity of the oil phase is reduced to a point below that of the aqueous phase to facilitate separation. In the first stage, a hydrocarbon diluent, such as benzene, toluene, gasoline, coal tar naptha, or petroleum naptha is employed. In the second stage, large quantities of water (2 volume of water per volume of oil) are added and the combined mass is subjected to a pressure separation, preferably centrifugation. This method is inadequate because of the large quantities of water required and the formation of oil-water emulsions which are very difficult to break. A smiliar water/hydrocarbon-diluent process is also disclosed in U.S. Pat. No. 3,553,098.
A non-aqueous recovery process is disclosed in U.S. Pat. No. 3,131,141, which process may be also characterized as a two-stage method since the tar sand is subject to contact with a series of solvents. The initial solvent contacting stage employs a light gas oil boiling within the range from about 400.degree. to 700.degree. F. to segregate a bitumen-gas oil phase and sand-gas oil phase. The separated sand-gas oil phase is then contacted with a liquified, normally gaseous hydrocarbon, preferably propane or butane, to separate a sand-liquified, normally gaseous hydrocarbon phase. Further separation steps produce sand, gas oil, and liquified, normally gaseous hydrocarbon. This process, while achieving good recoverey of the two solvents employed for extraction, has a relatively low rate of extraction and requires heat and pressure to effect the desired separations.
Sands with solvent recovery by steam stripping the separated sand is disclosed in U.S. Pat. No. 3,475,318. Prior methods has taught the complete removal of tar from the sand, but it was recognized that complete removal may be disadvantageous in that additional processing steps are required when refining the extracted tar to obtain the desired oil products. Specifically, the extracted tar must go through a flash distillation step in which undesirable oil products could selectively be removed from the tar sands, leaving the undesirable asphaltenes on the sand, by solvent extraction with a solvent consisting of a saturated hydrocarbon or mixture of saturated hydrocarbons having from 5 to 9 carbon atoms per molecule. Recognizing that it may be desirable to remove a certain portion of asphaltenes from the tar sand to use as fuel for the process, U.S. Pat. No. 3,475,318 further discloses the use of a controlled amount of an aromatic solvent having from 6 to 9 carbon atoms per molecule to extract the desired amount of asphaltenes. U.S. Pat. No. 3,079,326 discloses a double-solvent system (i.e., naphtha/methanol) for fractionating tar products obtained from low-temperatures pyrolysis of coal. The methanol solvent emloyed is an aqueous solution containing from 20 to 60 percent water which is used in a ratio varying from 0.5 to 2.0 volumes per volume of coal tar.